1. Field of the invention
The present invention relates to an optical system (hereinafter referred to as the focus extending optical system) for extending depth of field of optical lens(es) and an EDoF imaging system for performing a restoration process (deconvolution process) on an image, captured with the focus extending optical system, to sharpen the image.
2. Description Related to the Prior Art
Mobile phones, PDAs, and small notebook computers have normally been equipped with digital cameras. Generally, for example, a fixed-focus optical lens has been used for the digital camera installed in the mobile phone or the like to reduce the size and cost of the digital camera. Such digital camera has not been required to capture high quality images. Recently, however, even the simple digital camera has been required to improve image quality.
The digital camera installed in the mobile phone or the like is used for image capture of an object at a distance of several ten centimeters to several meters. To be more specific, the digital camera is used for imaging an object such as a person or a landscape several meters away from the digital camera. The digital camera is also used for reading characters, a two-dimensional code, or the like several ten centimeters away from the digital camera. To capture images at object distances in a wide range, the digital camera is required to focus in accordance with the object distance. However, it is difficult to provide a focus adjustment mechanism in the digital camera installed in the mobile phone or the like because the digital camera needs to be compact and inexpensive.
For this reason, in a device such as the mobile phone, an EDoF (Extended Depth of Field) imaging system has been employed as a digital camera which covers object distances ranging from a macro imaging range in the order of several ten centimeters to approximate infinity with no focus adjustment in view of cost and the like (see U.S. Pat. No. 8,294,764 (corresponding to Japanese Patent No. 2010-213274), Japanese Patent No. 2007-206738, and Japanese Patent No. 2006-094471). The EDoF imaging system captures an image with the use of a focus extending optical system. The focus extending optical system uses a phase plate or the like to have focal lengths that differ in accordance with a distance (incidence height) from an optical axis. The EDoF imaging system sharpens a captured blurred image through a restoration process. Thereby an image equivalent to that captured with optical lens (es) with wide depth of field is obtained.
When the EDoF imaging system captures an image of characters, a two-dimensional code, or the like, being an object at a near view, it is necessary that the obtained image allows not only human eyes but also software such as an OCR (Optical Character Recognition) application or a decode application to correctly recognize the characters or the like. People and software such as the OCR recognize characters or the like differently. The software may not correctly recognize the characters or the like in the image even if they are legible to the human eyes.
A conventional EDoF imaging system has extended depth of field, so that the characters or the like in an image (hereinafter referred to as the near view image) of a near view are legible to people. However, it is difficult for the conventional EDoF imaging system to capture an image of characters or the like which are accurately recognizable to the software such as the OCR.
It is necessary to improve the so-called image quality such as contrast and resolution to allow the software such as the OCR to recognize the characters or the like in the near view image. However, the image quality and the depth of field are in a trade-off relationship in the EDoF imaging system. Accordingly, it is necessary to reduce the depth of field to improve a character recognition rate of the software. This impairs the original function of the EDoF imaging system, that is, to extend the depth of field.